This invention relates to the to the automated transport of fluids.
Fluid dispensers are integral components of most automated clinical analyzers. U.S. Pat. No. 4,794,085 to Jessop proposes an apparatus and method for detecting sample aspiration in such instruments. The device and method employ a pressure sensor to detect the presence of the fluid meniscus in the sample container. When the meniscus is sensed it is assumed that fluid lies beneath and can then be aspirated and dispensed. This method and device have proven useful. However, fluids that are to be aspirated do not always present a meniscus that can reliably be used to determine the location of the surface of the fluid. When this happens, such analyzers can indicate that sufficient fluids have been aspirated when such is not the case. Accordingly, a method for indicating when such a false reading has occurred would be useful.
The invention is a method for determining whether a fluid has been aspirated. In the method, pressure readings are taken during a slow aspirate process and during a priming process. Reference pressure measurement also occurs after priming. Two differences are determined, one is the difference between pressure readings during slow aspirate and the reference pressure and the other is the difference between the reference pressure and pressure readings during prime. If either is less than a predetermined threshold then an error message is communicated.
In a further embodiment of the invention, the pressure reading during slow aspirate is a trough reading and the pressure reading prime is a prime reading where trough and peak values may be statistical (e.g. numerically averaged values) at or near the trough and peak readings.
In yet a further embodiment of the invention, the threshold is determined statistically. In this embodiment, a parameter such as CpK can be used to calculate the effectiveness that the film of fluid will be detected without a high frequency of false positive.